A thermal recording material is, in general, a material comprising a heat-sensitive color forming layer which comprises a colorless or light-colored electron-donating dye precursor and an electron-accepting color forming substance as the main components and is disposed on a support. The dye precursor and the color forming substance instantaneously react with each other by heating with a heated head, a heated pen or a laser beam, and a recorded material can be obtained. The thermal recording material described above has been developed for a long time. For example, as the heat-sensitive sheet for copying obtained by coating a paper with a specific coating layer comprising a composition which is colorless under the normal condition and develops color by heating or irradiation with infrared light, a heat-sensitive sheet for copying comprising a colorless dye base of the lactone, lactam or sultone type, an organic acid and a substance melting under heating as the reactive color forming component, is proposed (Patent Reference 1). As the thermal recording material which exhibits improved moisture resistance and stability during printing and can prevent coloring during drying and preparation of a coating component for forming records due to the improved moisture resistance, a thermal recording material in which the unit for forming records comprises a support sheet material comprising crystal violet lactone and a phenolic substance, the phenolic substance is a solid substance at the room temperature and liquefied or vaporized at the temperature of a thermograph to form records by the reaction with the lactone, and the lactone and the phenolic substance are dispersed in polyvinyl alcohol, is proposed (Patent Reference 2).
The thermal recording materials described above have advantages in that records can be obtained by relatively simple apparatuses, maintenance is facilitated, and generation of noise is suppressed and are utilized for thermal printers of various portable terminals, medical image printers attached to ultrasonic echo instruments and the like, thermopen recorders of cardiographs and analytical instruments, air plane tickets, train tickets and PUS labels for merchandises.
Various properties such as excellent color forming property, color formation in a great density with small amount of heat, excellent storage properties of the obtained images and maintenance of whiteness in undeveloped portions, are required for a thermal recording material. In particular, excellent storage properties such as excellent oil resistance, moisture resistance and heat resistance are required for labels for food processed by microwave ovens, parking tickets, delivery labels and tickets since reliability of recorded images is important Therefore, various compounds have been examined as the developer in thermal recording materials.
For example, thermal recording materials comprising α,α′-bis[4-(p-hydroxyphenylsulfonyl)phenoxy]-p-xylene, α,α′-bis[4-(p-hydroxyphenylsulfonyl)phenoxy]-m-xylene or α,α′-bis[4-(p-hydroxy-phenylsulfonyl)phenoxy]-o-xylene are proposed as the color developer providing a thermal recording material which exhibits great sensitivity, suppresses fog in the background and exhibits excellent storage properties, in particular, excellent resistance to water and plasticizers (Patent Reference 3). As the color developer for thermal recording materials exhibiting excellent storage stability of images obtained by the color formation, in particular, excellent resistance to plasticizers, oils, light and moisture under heat, diphenylsulfone bridged compounds as the reaction products of dihydroxydiphenylsulfone and alkylene dichlorides or α,α′-dichloroxylene are shown as examples (Patent Reference 4). However, the heat resistance of undeveloped portions is insufficient in the thermal recording materials described above, and the requirement for the excellent storage properties of developer is not sufficiently satisfied,    [Patent Reference 1] Japanese Patent Application Publication Showa 43(1968)-4160    [Patent Reference 2] Japanese Patent Application Publication Showa 45(1970)-14039    [Patent Reference 3] Japanese Patent Application Laid-Open No. Heisei 7(1995)-149713    [Patent Reference 4] Japanese Patent Application Laid-Open No. Heisei 10(1998)-29969